Conventionally, in many cases, an axial-type hydraulic piston pump driven by an engine, and an axial-type hydraulic piston motor driven by a high-pressure working oil are used in construction machines, for example.
For example, the axial-type hydraulic piston pump includes a cylinder block, a plurality of pistons, and a valve plate. The cylinder block is provided so as to rotate integrally with a rotation shaft rotatably provided inside a case and is formed with a plurality of cylinders that are circumferentially apart from each other and extend axially. Each of the pistons is slidably fitted into the corresponding cylinder of the cylinder block, and sucks/discharges a working oil by moving axially in accordance with the rotation of the cylinder block. The valve plate is provided between the case and an end surface of the cylinder block, and is formed with a suction port and a discharge port that communicate with each of the cylinders. In the hydraulic piston pump, when a drive shaft is driven to rotate, the cylinder block, together with a working shaft, rotates inside the case, and each of the pistons reciprocates in the corresponding cylinder of the cylinder block. The working oil sucked into the cylinders from the suction port is pressurized by the pistons, and is discharged from the discharge port as a high-pressure working oil.
When the cylinder port of each cylinder communicates with the suction port of the valve plate, a suction process is performed. In the suction process, the pistons move between the start edge and the end edge of the suction port in a direction projecting from the cylinders, and suck the working oil into the cylinder from the suction port. When the cylinder port of each cylinder communicates with the discharge port, on the other hand, a discharging process is performed. In the discharging process, the pistons move between the start edge and the end edge of the discharge port in a direction entering into the cylinders, and discharge the working oil inside the cylinders into the discharge port. By rotating the cylinder block so as to repeat the suction process and the discharging process, the working oil sucked into the cylinders from the suction port during the suction process is pressurized and discharged to the discharge port during the discharging process.